Gear pumps of this type are known, for example, from U.S. Pat. Nos. 2,140,966 and 3,118,387 as well as British Pat. No. 388,767. They generally comprise a ring rotatable about the axis of a substantially cylindrical housing chamber between two limiting positions about 180.degree. apart. The ring has an eccentric inner periphery defining a circular cutout within which an annular outer rotor with internal gear teeth is freely rotatable about another axis. An inner rotor has external gear teeth numbering one less than the internal gear teeth of the outer rotor and meshing therewith for entraining the latter when the inner rotor is driven by an outside prime mover. With the inner rotor centered on the chamber axis, the inter-teeth spaces formed by the two rotors vary in volume, increasing and decreasing in respective zones which lie on opposite sides of a plane of symmetry defined by the two axes. With a given direction of rotation, the ring surrounding the rotors occupies a limiting position in which the inlet and outlet ports respectively register with the zones of increasing and decreasing inter-teeth spaces so that the working fluid is pumped from the inlet port to the outlet port. When the direction of rotation is reversed, the ring must rotate into its alternate limiting position to maintain the same relationship between the ports and the zones of varying tooth spacing so that the working fluid continues to flow in the proper direction.
Various means are known for performing the switchover between the two limiting ring positions. Essentially, the ring is frictionally entrained by the outer rotor until it comes to rest against a suitable stop after half a revolution. In order to reduce the frictional resistance exerted upon the outer rotor by the arrested ring, the aforementioned U.S. Pat. No. 3,118,387 proposes to split that ring into two interconnected halves which are wedged apart by a lever coming to rest against a stop in either limiting position. A brochure published in 1969 by the assignee of that patent, entitled "Gerotor Standards", describes a spring-loaded slider inserted between the outer rotor and the eccentric ring which reduces--but does not eliminate--the frictional contact between the relatively rotating elements. German printed specification No. 2,055,883 describes a slider of this type which during steady-state operation is removed by fluid pressure from contact with the outer rotor against which it is held by a spring force during the switchover.